Polyvinyl alcohol is a kind of polymer widely used in the chemical industry; however, the manufacturing process of polyvinyl alcohol is always accompanying with the abundant production of the by-product, methyl acetate. Methyl acetate is a less valuable solvent due to its low industrial application and low volatility, even if the amount of the produced methyl acetate is one-half times higher than that of polyvinyl alcohol. Accordingly, the impure methyl acetate is usually discharged into the atmosphere after scrubbing in a wastewater treatment system or burned in an incinerator. In view of the above, if methyl acetate could be recycled to be efficiently transesterified into other kinds of esters having higher economical value, the working performance of the polyvinyl alcohol plants will be highly increased. In recent years, the transesterification processes of methyl acetate have been developed as below.
(1) Luyben et al (2004) proposed one transesterification process of methyl acetate. The transesterification process was performed by combination of a reactive distillation tower, a conventional distillation tower and a distillation tower of high pressure. Firstly, the conventional distillation tower has two feeding stream of methyl acetate; one is the mixture of methyl acetate and methanol, and the other is the reflux stream of the abundant methanol and methyl acetate from the top of the reactive distillation tower. The conventional distillation tower functions as separating methyl acetate from methanol. Methanol could be thus obtained from the bottom of the conventional distillation tower, whereas the azeotrope of methyl acetate and methanol could be formed on the top thereof.
The reactive distillation tower has also two feeding stream; one is the azeotrope, and the other is the reflux stream from the top of the distillation tower of high pressure, where the major composition of the stream is n-butanol. While the reflux of n-butanol is fed into the reactive distillation tower, another substream of fresh n-butanol is mixed simultaneously therewith to transfer into the upper portion of the reactive section in the reactive distillation tower, so that methyl acetate and n-butanol will react therein. Subsequently, one mixture of the abundant methanol and methyl acetate would be formed on the top of the reactive distillation tower, and then the mixture would be further refluxed into the conventional distillation tower to separate methyl acetate from methanol. Simultaneously, another mixture of n-butanol and butyl acetate would be obtained from the bottom of the reactive distillation tower, and the mixture is further introduced into the distillation tower of high pressure to separate n-butyl ester from butyl acetate.
The distillation tower of high pressure could break through the co-boiling point of n-butyl ester and butyl acetate by means of applying high pressure. Accordingly, the n-butanol could be formed on the bottom of the distillation tower of high pressure, whereas the substantial n-butanol is obtained from the top thereof, followed by refluxing the substantial n-butanol to the reactive distillation tower. Though the transesterification process of methyl acetate proposed by Luyben et al overcomes the co-boiling phenomenon of n-butyl ester and butyl acetate by means of the distillation tower of high pressure, the consumption of the steam amount of the distillation tower of high pressure brings about the considerable investing production cost to the conventional polyvinyl alcohol plants.
(2) Steinigeweg et al (2004) also proposed another transesterification process of methyl acetate. The process was performed by combination of a reactive distillation tower, a conventional distillation tower and a pervaporation device. The reactive distillation tower has two feeding streams; one is methyl acetate introduced from the lower portion of the reactive section thereof, and the other is n-butanol introduced from the upper portion of the reactive section thereof. Accordingly, the transesterified product, butyl acetate, would be obtained from the bottom of the reactive distillation tower and the mixture of methyl acetate and methanol would be formed on the top thereof.
Subsequently, the mixture is further introduced to the pervaporation device. By means of the pervaporation device, methyl acetate and methanol could be separated and respectively obtained with high purity. The separated methyl acetate with high purity as well as another fresh feeding stream of methyl acetate would be mixed and then introduced into the lower portion of the reactive section of the reactive distillation tower. Simultaneously, the separated methanol with high purity is introduced into the conventional distillation tower for a further separation, wherein the conventional distillation tower, methanol will be deposited on the bottom thereof, whereas the azeotrope of methyl acetate and methanol is formed on the top thereof. Then, the azeotrope is refluxed to the pervaporation device for a further separation of methyl acetate and methanol.
Though the transesterification process proposed by Steinigeweg et al overcomes the co-boiling phenomenon of methyl acetate and methanol by means of the pervaporation device, the regular placement of the film in the pervaporation device also lowers down the overall economic efficiency of the conventional polyvinyl alcohol plants.
In view of the mentioned drawbacks, a special transesterification process of methyl acetate with the competitive investing production costs and the higher conversion rate of methyl acetate is necessary for most polyvinyl alcohol plants.
From the above description, it is known that how to develop an efficient and economical transesterification process of methyl acetate has become a major problem to be solved. In order to overcome the drawbacks in the prior art, an improved transesterification process of methyl acetate is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the invention has the utility for the industry.